Regan Brown
Fuel injectors are simple devices that have been around since the 1960s, but they can still baffle technicians when problems arise. An ohmmeter can be used to test for resistance in the coil of the injector, which can indicate whether the injector is weak or has shorted. However, this is only an electrical test and will not show other problems with the injector. A more conclusive test is to measure the inductance and coil Q with an impedance bridge.
| Characteristics | Values |
|---|---|
| What an ohm meter measures | Resistance |
| What will damage an ohm meter | Any voltage running through the circuit |
| How to isolate injectors | Put the ohm meter across each set of leads |
| What a measurement of infinity on the ohm meter means | Open circuit and the coil in the injector is bad |
| What a measurement of 0 on the ohm meter means | The solenoids are fine |
| What a measurement of 0.002 on the ohm meter means | The solenoids are fine |
| What resistance measurements can show | Gross failures of the coil in the injector |
| What a short to the adjacent turn will not be noticed with | An ohmeter |
| A more conclusive test | Measure the inductance and coil "Q" with an impedance bridge |
| Who has access to an impedance bridge | Most car enthusiasts do not |
| When to check resistance | After the injector is up to operating temperature |
| What the resistance should be | About 16 ohms |
| When to replace or recondition the injector | When the resistance gets down below the teens |
| What a resistance check will show | A weak injector (resistance too low) which has shorted windings or one that has an open in its windings (infinite resistance) |
| How to check for a leak in the fuel system | Put a gauge on the Schraeder valve at the fuel rail and watch the leak down rate |
| How to test the injectors | Pull them off and install them on a test bench where you can supply high-pressure fuel, power the injectors, and test fire them looking for leakage, and spray pattern |
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What You'll Learn
Checking for a short circuit
Firstly, using a voltmeter, adjust the setting to ohms. Then, expose the injector electrical terminals and connect the voltmeter leads to the terminals. The polarity of the leads is irrelevant. This test gives a baseline ohm reading of all injectors, and a service manual will also have this information. Most injector readings should range between 11 and 14 ohms.
If your ohm meter measures infinite resistance, this indicates an open circuit, and the coil in the injector is bad. If the resistance is too low, this indicates a weak injector, which has shorted windings. In either case, the injector will need to be replaced.
It is important to note that resistance measurements can only show gross failures of the coil in the injector. A short to the adjacent turn will not be noticed with an ohm meter, but it will definitely affect its performance. A more conclusive test is to measure the inductance and coil "Q" with an impedance bridge.
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Testing for a fuel leak
Firstly, identify the location of your fuel injectors. Typically, there is one injector per cylinder, usually a black plastic housing installed directly above the cylinder. With your vehicle turned off, disconnect the electrical connector for the fuel injector you want to test.
Next, use an ohm meter or a multimeter to take a resistance reading. An ohm meter measures resistance, and any voltage running through the circuit will damage the meter, so ensure you only connect it once the vehicle is off. You can use the ohm meter to measure the coil resistance of the injectors. The resistance of the coils should be within about plus or minus 10% of each other. Most fuel injectors should read between 10 and 18 ohms, but check your vehicle's service manual or look online for the correct value.
If your injector is leaking, the ohm meter may show an infinite reading, indicating an open circuit, or a very high resistance reading. A reading of zero resistance means the coil is shorted. If your readings are off, the faulty injector needs to be repaired or replaced.
It is important to note that an ohm meter test only verifies that the injectors are electrically capable of working. To properly test for a fuel leak, you should pull the injectors off and install them on a test bench where you can supply high-pressure fuel, power the injectors, and test fire them, looking for any leakage.
Additionally, you can check for a leak in the fuel system by putting a gauge on the Schrader valve at the fuel rail and observing the leak-down rate. Another method is to put your truck in neutral, bring the RPMs to the redline for a few seconds, and then let off. If the RPMs stay up, it could indicate that the injectors are leaking.
If you suspect your fuel injectors are causing problems due to dirt, carbon buildup, or a worn or broken valve, you can use a commercial injector cleaner or a high-quality cleaning fuel system additive to fix the issue.
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Measuring coil resistance
To measure the coil resistance of fuel injectors, you will need an ohm meter or a multimeter. An ohm meter measures resistance, and any voltage running through the circuit will damage the ohm meter, so it is important to isolate each injector and put the ohm meter across each set of leads. If the ohm meter measures infinity, this indicates an open circuit, and the coil in the injector is bad.
It is important to note that resistance measurements can only show gross failures of the coil in the injector. A short to the adjacent turn will not be noticed with an ohm meter but will affect its performance. A more conclusive test is to measure the inductance and coil "Q" with an impedance bridge. An impedance bridge measures the A.C. impedance of coils and capacitors (reactance and resistance) and will also measure pure resistance.
To check the coil resistance of fuel injectors with an ohm meter or a multimeter, follow these steps:
- Disconnect the injector from the fuel rail and ECM connector.
- Set the multimeter to the ohms setting (resistance).
- Touch the two metal contacts on the injector with the two prongs of the multimeter.
- The resistance of the coils should be within about plus or minus 10% of each other. For example, if one vehicle's injectors measure about 12 ohms, and one measures 8 ohms, the last one probably has shorted turns and needs replacement.
- If the resistance is too low, it indicates a weak injector with shorted windings.
- If the resistance is infinite, it indicates an open in the windings, and the injector will need to be replaced.
It is important to note that this is only an electrical test. If the internal parts of the injector are worn or leaking, the injector would test good with an ohm meter if the coil is good. A more comprehensive test would be to pull the injectors off and install them on a test bench where you can supply high-pressure fuel, power the injectors, and test fire them, looking for leakage and spray pattern.
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Checking for worn injectors
A more comprehensive approach to checking for worn injectors involves removing them and installing them on a test bench. This allows for high-pressure fuel supply, powering the injectors, and testing for leakage and spray pattern. Another method is to put the vehicle in neutral, bring the RPMs to the redline for a few seconds, and then let off. If the RPMs hang periodically during the drop, it indicates that the injectors may be worn.
Additionally, resistance measurements can be used to identify weak injectors with shorted or open windings. A resistance check will show if the resistance is too low or infinite, respectively. It is recommended to measure the resistance when the injector is at operating temperature. The resistance values should be within a certain range, typically around 10-16 ohms, and any significant deviations may indicate the need for replacement or reconditioning.
It is worth noting that even with modern gasoline additives to prevent deposit formation and clogging, fuel contamination and high mileage can still lead to injector blockage. Therefore, regular cleaning and maintenance of fuel injectors are essential to ensure optimal performance.
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Using an impedance bridge
To check your fuel injectors with an impedance bridge, first, allow your engine to reach operating temperature. Then, take impedance readings from the fuel injectors. These should be around 16 ohms, and if they fall below the teens, your fuel injectors may be faulty and should be replaced or reconditioned.
It is important to note that resistance measurements can only show gross failures of the coil in the injector. A more conclusive test is to measure the inductance and coil "Q" with an impedance bridge. This will provide a more comprehensive understanding of the condition of your fuel injectors.
Additionally, it is recommended to consult your vehicle's service manual or online resources to determine the correct resistance values for your specific fuel injectors. Most fuel injectors should have a resistance reading between 10 and 18 ohms, and all injectors should have readings within the range suggested by the manufacturer. If any resistance readings are off, the faulty injector needs to be repaired or replaced.
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Frequently asked questions
First, isolate each injector and put the ohm meter across each set of leads. If the ohm meter measures infinity, this indicates an open circuit and a bad coil in the injector.
Resistance measurements can indicate gross failures of the coil in the injector. Low resistance indicates a weak injector, while infinite resistance indicates an open in its windings.
An ohm meter only measures DC resistance. It cannot detect a short to the adjacent turn, which will affect the injector's performance.
